In 1978 Abrahamson et al reported finding nanoscale hollow fibrils or tubular fibres on anodes after an arc discharge in nitrogen—Abrahamson J., Wiles P, “Carbon fibre layers on Arc electrodes—their Properties and Cool-Down Behaviour”, Carbon, vol 16, 341-349, (1978). In 1991 S. Iijima published a paper which brought the nanotubes to the, world's attention—Iijima S, “Helical Microtubules of Graphitic Carbon,” Nature, vol 354, 56-58, (1991). Since then there has been much work as evidenced by the scientific and patent literature on the study of these nanotubes, methods for producing them, and applications.
A variety of different synthesis methods are used to produce carbon nanotubes. The three most commonly used methods are the arc discharge, laser ablation, and chemical vapour deposition techniques. In the arc-discharge method current flows between closely spaced carbon electrodes creating an arc between them. The arc temperature is in excess of 6000 k and the electrodes reach a temperature of about 4000K at which the carbon of the electrodes strongly vaporises. Evaporation of the carbon electrodes form a vapour which condenses as nanoscale carbon fibrils or nanotubes, which are collected after the arc has been extinguished. The electrodes are usually high purity graphite rods of diameter from 3 to 15 mm. The arc discharge is created within a vacuum chamber such as a stainless steel chamber with viewing ports. The chamber is connected to a vacuum pump and a gas supply. A continuous flow of gas which is commonly helium is preferred over a static atmosphere. As the electrodes have to be adjusted during each run to create and maintain the arc a mechanism for this is required. In addition often the electrodes are cooled with water. The nanotubes are formed within the core of the electrode slag. This is collected and refined. The core generally also includes different contaminating materials (nano-onions, amorphous carbon and fullerenes) and often needs to be treated to gain product purity.
The nanotubes consist of carbon atoms mainly arranged in hexagons. There are two kinds of nanotubes: single walled nanotubes (SWNT) and multiwalled nanotubes (MWNT). Both can be grown in tangled structures or ordered close-packed structures. The mechanical and electrical properties of carbon nanotubes make them suitable for many applications: their length and flexibility makes them suitable for use as nanoscale tweezers and improves the precision of scanning probe microscopes; their ability to emit electrons has proved to be useful for many applications such as in oriented regular arrays of carbon nanotubes for operating as a flat screen colour display, in which nanotubes supply the electron beams that cause phosphor on the screen to light up; for building logic gates; for hydrogen fuel storage; and their stiffness and tensile strength may make them suitable for new composite materials.
In the arc discharge method for producing the nanotubes one or both of the electrodes may be enriched with a metallic catalyst(s). The presence of specific metal catalysts may determine whether SWNTs or MWNTs are produced, and the presence of an appropriate metal catalyst can also increase the yield of nanotubes. Ni—Co, Co—Y, Ni—Y catalysts are used in different variations.
Methods for the continuous production of carbon nanotubes are disclosed in: “A Simple Method for the Continuous Production of Carbon Nanotubes”, Ishigami et al, Chemical Physics Letters 319 (2000) 457-459; “Continuous Production of Aligned Carbon Nanotubes: A Step Closer to Commercial Realisation”, Andrews et al, Chemical Physics Letters 303 (1999) 467-474; and “Semi-Continuous Synthesis of Single-Walled Carbon Nanotubes via Hyrogen Arc Discharge Method”, Liu et al, Carbon 37 (1999) 1865-1868 and “Continuous Production of Carbon Nanotubes by Using Moving Bed Creator”, Liu et al, Chinese Chemical Letters 12 (12): 1135-1138, December 2001.
PCT patent application WO 01/85612 discloses a method in which carbon nanotubes are formed on a porous carbon substrate and in particular on carbon paper.
Cherrey et al in “Synthesis of BxCyNz Nanotubes”, Physical Review B, Vol 51, No. 16, 15 Apr. 1995 disclose forming nanotubes of stoichiometry BC2N and BC3.